Inter-kingdom signalling: communication between bacteria and their hosts

Abstract

Microorganisms and their hosts communicate with each other through an array of hormonal signals. This cross-kingdom cell-to-cell signalling involves small molecules, such as hormones that are produced by eukaryotes and hormone-like chemicals that are produced by bacteria. Cell-to-cell signalling between bacteria, usually referred to as quorum sensing, was initially described as a means by which bacteria achieve signalling in microbial communities to coordinate gene expression within a population. Recent evidence shows, however, that quorum-sensing signalling is not restricted to bacterial cell-to-cell communication, but also allows communication between microorganisms and their hosts.Prokaryotes and eukaryotes have coexisted for millions of years. It is estimated that humans have 10 13 human cells and 10 14 bacterial cells (comprising the endogenous bacterial flora). Eukaryotes have a variable relationship with prokaryotes, and these interactions can be either beneficial or detrimental. Humans maintain a symbiotic association with their intestinal microbial flora, which is crucial for nutrient assimilation and development of the innate immune system 1 . These mutually beneficial associations are possible because microorganisms and mammals can communicate with each other through various hormone and hormone-like chemical compounds. These signals, however, can be 'hijacked' by bacterial pathogens to activate their virulence genes.The hormonal communication between microorganisms and their hosts, dubbed inter-kingdom signalling, is a recent field of research. This field evolved from the initial observation that bacteria can communicate with each other through hormone-like signals 2 , a process that was later named quorum sensing (QS) 3 . This field expanded with the realization that these bacterial signals can modulate mammalian cell-signal transduction 4 and that host hormones can crosssignal with QS signals to modulate bacterial gene expression 5 .In this Review, we discuss several mechanisms that are used for hormonal communication between micro-organisms and their hosts. Owing to space constraints, we mainly consider pathogenic interactions. We focus primarily on acyl-homoserine lactones (AHLs) and aromatic (autoinducer (AI)-3) signals, because of the wealth of reports that link these signals to interkingdom communication, but it is worth noting that bacteria use an array of additional chemical

Incidence of bacterial pathogens in equine uterine swabs, their antibiotic resistance patterns, and selected reproductive indices in English thoroughbred mares during the foal heat cycle

“…Although most of the relevant studies were performed on human or laboratory female animals, all data suggest that these sex steroid hormones play a key role in the modulation of interactions between bacterial microorganisms and the host's environment (Garcia-Gomez et al 2013). This bacterial-host communication can mediate the activation of virulence factors of microbial pathogens thus affecting the pathogenesis and prognosis of infection in their host organism (Hughes and Sperandio 2008). From a patho-physiological point of view, the aforementioned mechanisms are only partially understood and require further research.…”

Taurocholic acid metabolism by gut microbes and colon cancer

“…38 It is interesting to note that only a small number of intestinal Clostridium spp. have evolved a multi-gene bile acid inducible operon capable of bile acid 7a-dehydroxylation of CA and CDCA and 7b-dehydroxylation of UDCA.…”

“…have evolved a multi-gene bile acid inducible operon capable of bile acid 7a-dehydroxylation of CA and CDCA and 7b-dehydroxylation of UDCA. 37,38,43 While the proximal explanation of this metabolic capacity is that the bile acid 7a/b-dehydroxylation pathway yields a net 2 electron reduction, the ultimate explanation might relate to production of a toxic, antimicrobial compound. 44,45 In addition, the fact that secondary bile acids (DCA, LCA and derivatives) are high affinity ligands to several host nuclear and G-coupled protein receptors suggests perhaps these metabolites are involved in interkingdom signaling.…”

“…To date, organisms that express BSH are not capable of converting CA to deoxycholic acid (DCA). 37,38 Likewise, organisms capable of producing DCA and LCA from host primary bile acids lack BSH and are incapable of converting conjugated primary bile acids such as TCA to DCA without the presence of BSH expressing taxa capable of converting TCA to free CA. 39,40 To survive in the gut environment, microorganisms have to contend with numerous host selection pressures, particularly anti-microbial agents such as lectins, defensins, and bile acids.…”

“…44,45 In addition, the fact that secondary bile acids (DCA, LCA and derivatives) are high affinity ligands to several host nuclear and G-coupled protein receptors suggests perhaps these metabolites are involved in interkingdom signaling. 19,37,38 During enterohepatic circulation, secondary bile acids are produced in the cecum and colon and returned to the liver by passive diffusion through the gut epithelium into the portal circulation. 39 Evidence is emerging that as animal-based diets stimulate more bile input into the gut the levels of DCA-producing bacteria increase.…”